After promotion on the replica to primary the new replica is in Replication Status unreachable.

Pre-check before promotion was ok, checked all (network, dns resolution, clock, port...), user are autenthicating on both instances. The old primary seems to be correctly demoted and it is shown as replica now.

When you promoted the original replica, did you promote for Maintenance? that type of promotion would attempt to attach the original primary as a replica, after copying over all log files. If you promoted for Disaster Recovery DR, then the original primary will never attach. If the Promotion was for Maintenance but the original primary failed to attach, there would be logs associated with that, and that is why Piers recommended opening a case. Some error messages could indicate port 7072 was not open between the Primary and the Replica. But there are other reasons it could have failed